1. Technical Field
This invention relates to cordless telephone systems and more particularly to systems that include security arrangements for preventing unauthorized use of such systems.
2. Description of the Prior Art
A typical cordless telephone system includes a handset or remote unit and a base unit. The base unit is connected to the telephone lines and includes an antenna, a transmitter and a receiver for communicating a radio frequency carrier signal that is modulated by switching signals and audio signals to and from the handset unit as appropriate. The handset unit includes a speaker and a microphone, and also an antenna, a transmitter and a receiver for likewise communicating telephone switching signals and auido signals to and from the base unit.
In the operation of a typical cordless telephone system, per the transmitter in the handset unit is enabled causing a control signal to be generated and transmitted to the the base unit. Upon receipt and detection of the control signal, the base unit seizes the telephone lines so as to enable audio signals and dialing signals from the handset unit that are received by the base unit within a given audio frequency band to be passed onto the telephone lines. While such an arrangement was found satisfactory while cordless telephones were few in number, it appears to be unacceptable today since there are only a few allocated frequencies for cordless telephone systems and many more cordless systems are now being employed. Because of this, many users are, in fact, assigned the same frequencies for transmitting and receiving. People cognizant of this fact have used the handset or remote unit to place unauthorized telephone calls over the telephone lines of other users.
In order to prevent the placing of unauthorized telephone calls, one widely used system in the known art utilizes a control code that is determined by switches manually preset in both the handset unit and the base unit. Only after receipt of a signal with this control code from the handset unit and a favorable comparison thereof with the control code stored by the base unit, does the base unit allow the telephone lines to be seized by the handset unit for placing a telephone call. The possibility of mismatching the common information in the two sets of switches is high, however, and the number of switches is therefore usually kept to a small number. This increases the opportunity for an unauthorized user to place telephone calls over the telephone lines of other cordless telephone system owners if he is able to discover the correct control code for causing a base unit to provide dial tone to the handset unit of his cordless telephone system. With the range of communication between a base unit and a handset unit easily being a few hundred feet, it is quite feasible that a person may travel around in an automobile with the handset unit until he receives dial tone and then place a call over another person's telephone lines.
In another security system for use with cordless telephones, preselected digits are set in the base unit by means of switches. The selected digits must then be dialed by the handset unit before dialing the desired telephone number. If a favorable comparison is had between the dialed digits and those digits set by the switches, the security system connects the telephone line to the base unit and allows dial tone to be transmitted thereby enabling a call to be implemented by the user. This system has improved security over some other known systems. It has the disadvantage, however, of requiring additional digits to be dialed before those of a desired telephone number each time a telephone call is made.
Still another system uses the radio frequency link between the base unit and the handset unit to set the security code. The security code is manually set in the base unit by means of switches and then automatically set in the handset unit via the radio frequency link by the base unit. In that the security code is transmitted from the base unit to the handset unit, reception of this code is quite possible for a would be unauthorized user who has knowledge of the operation of this type of cordless telephone system.
While the foregoing cordless telephone systems have been generally satisfactory in the past, it is now technically feasible and desirable to provide a cordless telephone system that provides increased security while being relatively inexpensive and easy to operate.